Are you aware of any downloadable collection of symbols and illustrations of use in ID?
Does "google" count as a recommendation?
I must admit I'm not a user of pre-made clipart. I guess before paying *anything* you should take a good look if it's not a bunch of lo-res jpegs, or the infernal file type WMF. EPS, in general, ought to be good, although "for best user experience" I always convert them to PDF by dropping them on the Distiller.
It is a rather vague request. Too vague for a "specific recommendation."
Office clip art? Religious? Animals? People? Silhouettes? Backgrounds? Nature? Design elements? Stock sites that charge? Free sites?
The list is nearly as vast as the internet.
Be specific in your Google search terms and you will find many, many hits.
Take care, Mike